1. Technical Field of the Invention
The present invention relates to a bolometer infrared detector that detects infrared rays with a thermal resistance element formed in a thermal isolation structure, and, more particularly, to a bolometer infrared detector comprising a part which reduces an afterimage and an afterimage reduction method which uses the bolometer infrared detector.
2. Description of the Related Art
The infrared detector is divided broadly into a quantum detector which uses a band structure of a semiconductor and so on, and a thermal detector which uses a heat-originated change in material properties such as a resistance and a dielectric constant. The former detector has high sensitivity but needs to be cooled according to the operational principles, whereas the latter, which is called an uncooled detector because it does not require cooling, has many advantages over the quantum detector in production cost and maintenance cost. Thus, the uncooled detector is becoming the mainstream of infrared detectors.
There are a bolometer detector, a pyroelectric detector, and a thermocouple detector as thermal infrared detectors. Generally, each type of detector has a thermal isolation structure, a so-called diaphragm structure, in order to enhance the sensitivity of the detector. The bolometer infrared detector is comparatively superior in its characteristics. Especially, bolometer infrared detectors using vanadium oxide (VOx) as a bolometer material are used as imaging devices which are arrayed in two dimensions to acquire infrared images, as shown in SPIE (year 1996, volume 2746, page 23). The above-described bolometer infrared detector will be explained with reference to FIGS. 1A and 1B. FIG. 1A is a cross sectional view showing an infrared detector 1. FIG. 1B is a cross sectional view showing an infrared detecting element 2 shown in FIG. 1A.
As shown in FIG. 1B, in the device section of the bolometer infrared detector, an infrared reflecting film 12 which reflects infrared rays and a protective film 13 are formed on a circuit substrate 11 in which a readout circuit 11a is formed by a CMOS process. On the infrared reflecting film 12, separated by a cavity 14, a temperature detector 19 comprising a bolometer thin film 16 made of a bolometer material, an electrode 17 connected to an end of the bolometer thin film 16, and a protective film 15 covering those mentioned is formed. The temperature detector 19 is held in air by beams 18 consisting of the electrode 17 and the protective film 15 covering the electrode 17. The electrode 17 is connected to a contact 11b on the circuit substrate 11, and the contact 11b is connected to the readout circuit 11a electrically.
As shown in FIG. 1A, the elements having such a thermal isolation structure are arrayed in a two-dimensional array to constitute the infrared detecting element 2. The infrared detecting element 2 is mounted on a temperature control element such as a Peltier device 3 and is controlled to a constant temperature. The infrared detecting element 2 and the Peltier device 3 are sealed in vacuum by a vacuum package comprising a package 4, a cap 5, an infrared-transparent window 6, and an exhaust pipe 7, and their input and output signals are connected via pins 8 to an external circuit of such as a driver, a signal processor, and a temperature control element driver provided outside.
Infrared rays made incident through an optical system are absorbed partly by the protective film 15 and the bolometer thin film 16 of each infrared detecting element 2, and the remainder passes through them and is reflected by the infrared reflecting film 12 to be made incident on the temperature detector 19 again and absorbed again. The absorbed infrared rays heat the temperature detector 19 and change the resistance of the bolometer thin film 16 (thermal resistance) in the temperature detector 19. As a bias current from the readout circuit 11a is supplied, the resistance change is read out as a voltage change so that the temperature of a subject is measured.
Japanese Published Unexamined Patent Application No. 2002-71452 discloses a thermal infrared detector having a similar bolometer thin film. In order to suppress the drift of the output of the detector due to an environmental temperature and self-heating of the detector, this thermal infrared detector comprises a first temperature detector and a second temperature detector at each of pixels arranged in an array, and both of the temperature detectors have the bolometer thin films which are formed floating from substrate by the beams so that both temperature detectors are thermally separated from the substrate. Eaves are provided so as to extend outward from both ends of the first temperature detector in such a way as not to touch the beams and the second temperature detector. The infrared reflector is formed on an upper layer of the second temperature detector. By setting thermal capacities of the eaves and the infrared reflector approximately equal to each other, the influence of the temperature change due to the environmental temperature and the self-heating of both temperature detectors is cancelled, so that only signals originated from infrared rays made incident on the first temperature detector and the eaves are taken out.
However, these conventional infrared detectors have problems as follows. It is generally known that an imaging device which images a subject with light receiving elements suffers a phenomenon that the image of a certain object imaged is also observed even after the object has been removed (the phenomenon will hereinafter be referred to as an afterimage) due to the characteristic of light receiving elements or the characteristic of a signal processing circuit. It is also known that with the use of the bolometer infrared detector 1, an afterimage appears at a place where an object with a certain temperature, especially a high temperature, is imaged. The time when the afterimage appears depends on the temperature of the high temperature object and the imaging time of the high temperature object. When a high temperature object of about 500 K is imaged for about 3 minutes, for example, several minutes are required for the level of the afterimage to become equal to or less than the NETD (Noise Equivalent Temperature Difference), raising a serious problem for a system which acquires a heat image in real time. Therefore, there is a demand for a bolometer infrared detector 1 which can reduce an afterimage.